The British Nutrition Foundation recommends that this group should make up one-third of the diet and that each meal should be based around one food from this group. Potatoes, yams, plantains and sweet potato are classified as ‘starchy carbohydrates’ and are, therefore, considered within this group rather than as fruit and vegetables. Other foods in this group include breakfast cereals, rice and noodles. These foods are sources of carbohydrate and fibre that provide sustained energy release. Some also contain iron and B-group vitamins including folic acid (p. 279).

Fruit and vegetables

Foods in this group include fresh, frozen and canned products, 100% fruit or vegetable juices and pure fruit smoothies. These foods provide carbohydrate, fibre, vitamin C, folic acid and fibre. A minimum of five portions per day is recommended.

In addition to the food shown in Figure 11.1, this group includes meat products such as bacon, sausages, beef­burgers, salami and paté. Moderate amounts are recommended because many have a high fat content. It is suggested that fish, including one portion of oily fish, e.g. salmon, trout, sardines or fresh tuna, is eaten twice weekly. This food group provides protein, iron, vitamins B and D and sometimes minerals. Vegetar­ian alternatives include tofu, nuts, beans and pulses, e.g. lentils. Beans and pulses are also a good source of fibre.

Foods and drinks high in fat and/or sugar

These foods are illustrated in Figure 11.1 and also include oils, butter, margarine (including low-fat spreads), mayonnaise, fried food including chips, crisps, sweets, chocolate, cream, ice cream, puddings, jam, sugar and soft drinks, but not diet drinks. Fats are classified as saturated or unsaturated and the differences between these are explained on page 277. Foods in this group should only be used sparingly, if at all, as they are high in energy and have little other nutritional value.

Additional recommendations

The British Nutrition Foundation makes other specific recommendations about salt (p. 280) and fluid intake (1.5–2 L per day). This includes water, tea, coffee, squash and fruit juice. Alcohol intake should not exceed 3–4 units per day for men and 2–3 units per day for women.

Certain groups of people require a diet different from the principles outlined above. For example, pregnant and lactating women have higher energy requirements to support the growing baby and milk production. Menstruating women need more iron in their diet than non-menstruating women to compensate for blood loss during menstruation. Babies and growing children have higher energy requirements than adults because they have relatively higher growth and metabolic rates. In some gastrointestinal disorders there is intolerance of certain foods, which restricts dietary choices, e.g. coeliac disease (p. 331).

Digestion, absorption and use of nutrients are explained in Chapter 12. Structures of carbohydrates, proteins and fats are described in Chapter 2.

Carbohydrates are digested in the alimentary canal and absorbed as monosaccharides. Examples include glucose (see Fig. 2.7, p. 26), fructose and galactose. These are, chemically, the simplest form of carbohydrates.

Disaccharides

These consist of two monosaccharide molecules chemically combined, e.g. sucrose (see Fig. 2.7, p. 26), maltose and lactose.

Polysaccharides

These are complex molecules made up of large numbers of monosaccharides in chemical combination, e.g. starches, glycogen and cellulose.

Not all polysaccharides can be digested by humans; e.g. cellulose and other substances present in vegetables, fruit and some cereals pass through the alimentary canal almost unchanged (see NSP, p. 281).

• provision of energy and heat; the breakdown of monosaccharides, preferably in the presence of oxygen, releases heat and chemical energy for metabolic work – glucose is the main fuel molecule used by body cells

• ‘protein sparing’; i.e. when there is an adequate supply of carbohydrate in the diet, protein does not need to be used to provide energy and heat, and is used for its main purpose, i.e. building new and replacement body proteins

• providing energy stores when carbohydrate is eaten in excess of the body’s needs as it is converted to:

– glycogen – as a short-term energy store in the liver and skeletal muscles (see p. 315)

– fat, that is stored in adipose tissue, e.g. under the skin.

Amino acids (see Fig. 2.8)

These are composed of the elements carbon, hydrogen, oxygen and nitrogen. Some contain minerals such as iron, copper, zinc, iodine, sulphur and phosphate. Amino acids are divided into two categories: essential and non-essential.

Essential amino acids cannot be synthesised in the body, therefore they must be included in the diet. Non-essential amino acids are those that can be synthesised in the body. The essential and non-essential amino acids are shown in Box 11.2.

Excess amino acids are broken down. The amino group (~NH₂) is converted to the nitrogenous waste product urea and excreted by the kidneys. The remainder of the molecule is converted to either glucose or a ketone body (see ketosis, Ch. 12), depending on the amino acid. Negative nitrogen balance occurs when amino acid supply does not meet body needs. This situation may arise either when dietary protein intake is inadequate, e.g. deficiency or absence of amino acids, or protein requirement is increased, e.g. during growth spurts and following injury or surgery.

Biological value of protein

The nutritional value of a protein, its biological value, is measured by how well it meets the nutritional needs of the body. Protein of high biological value is usually of animal origin, easily digested and contains all essen­tial amino acids in the proportions required by the body.

A balanced diet, containing all the amino acids required, may also be achieved by eating a range of foods containing proteins of lower biological values, provided that deficiencies in amino acid content of any one of the constituent proteins of the diet is supplied by another. A balanced vegetarian diet consists primarily of proteins with lower biological values, e.g. vegetables, cereals and pulses. When proteins from different plant sources are combined, they complement each other providing higher biological values than a single plant source. Through this complementary action the biological value of vegetarian diets can be similar to those based on animal protein.

• growth and repair of body cells and tissues

• synthesis of enzymes, plasma proteins, antibodies (immunoglobulins) and some hormones

• provision of energy. Normally a secondary function, this becomes important only when there is not enough carbohydrate in the diet and fat stores are depleted.